1. Field of the Invention
The present invention relates to a motor control device having a function of detecting a magnetic-pole position of a permanent magnet synchronous motor in a sensor-less manner.
2. Description of the Related Art
As a control method for a permanent magnet type synchronous motor, for example, a three-phase synchronous motor, a control method called vector control is known. In the vector control, a motor current flowing into three-phase stator coils of a motor is coordinate-transformed into vector components along a d axis, which lies in a direction of a magnetic field generated by a permanent magnet disposed in a rotor, and a q axis orthogonal to the d axis, whereby feedback control is carried out. For the coordinate transformation, it is necessary to highly precisely detect the position of the rotor (magnetic-pole position). For the magnetic-pole position detection, a rotation sensor such as a resolver is used. A sensor-less magnetic-pole detection technology of electrically detecting a magnetic-pole position by utilizing counter-electromotive force deriving from rotation of the rotor is also available. However, when the motor is stopped, since the counter-electromotive force is not developed, a high-frequency current or a high-frequency voltage is applied to the motor. Based on the response, the magnetic-pole position is estimated.
In JP-A-2008-79489, a motor control device having such a sensor-less magnetic-pole detecting function has been disclosed. According to the publication, an error between a magnetic-pole position obtained based on a q-axis current detected by applying an alternating voltage to a motor, and an estimated magnetic-pole position is corrected in order to obtain the magnetic-pole position. Further, a d-axis voltage command is obtained based on a d-axis current command on which a d-axis bias current of a constant wave that alternates positively and negatively symmetrically is superposed, and the d-axis voltage command is applied to the motor. The direction of a magnetic pole of a permanent magnet is identified based on the relationship of large and small magnitudes of the d-axis bias current between when the d-axis bias current is increased at the positive-to-negative switching timing of the d-axis bias current and when the d-axis bias current is decreased thereat.
The motor control device described in JP-A-2008-79489 has an excellent sensor-less magnetic-pole detection function capable of identifying the position of a magnetic pole as well as the polarity of the magnetic pole. However, at the positive-to-negative switching timing of the d-axis bias current, there is a possibility that a d-axis current may be in a transient state and a d-axis voltage may not be stable. As a result, there arises a possibility that the stability in identifying the polarity of the magnetic pole may be impaired and erroneous detection may be invited.
Therefore, it is desired that when a permanent magnet type synchronous motor is stopped, the polarity of a magnetic pole of the motor is stably identified without use of a rotation sensor.